1. Field of the Invention
The present invention relates to a suspension system of a motor vehicle.
2. Description of the Related Art
Today, motor vehicles, such as minivans and wagons, generally employ in their rear suspension systems a structure in which coil springs are disposed separately from shock absorbers, or dampers, and mounted directly on lower suspension arms having an I-shaped cross section as shown in Japanese Unexamined Patent Publication No. 2002-293120 to provide as large an in-vehicle space as possible, rather than a structure in which each coil spring surrounds a damper in a coaxial configuration. In this kind of motor vehicles, great emphasis is placed on their performance to give a comfortable ride. To ensure increased riding comfort, it has been necessary to make the length of the coil springs as large as possible.
In the aforementioned structure in which the coil springs are mounted directly on the lower arms, the height of a floor of the vehicle increases if the length of the coil springs is increased and, therefore, it has been essential to give particular consideration to vehicle design to provide a large in-vehicle space. To achieve this design objective, the aforementioned Japanese Unexamined Patent Publication No. 2002-293120 proposes an independent suspension type rear suspension system employing a specially designed structure including coil springs placed between a vehicle body and left and right lower arms having an I-shaped cross section in which an upward opening dish-shaped (or recessed) coil spring seat portion is formed in each of the lower arms at about the middle of the length thereof for supporting the coil springs. Provided with the recessed coil spring seat portion in each of the lower arms, this lower arm structure is intended to locate lower ends of the coil springs as low as possible so that the length of the coil springs can be increased. This approach is now widely taken for granted in the aforementioned kind of motor vehicles.
Even if the length of the coil springs is increased by forming the recessed coil spring seat portions in the lower arms for supporting the coil springs as mentioned above, however, it is actually impossible to achieve a desired level of ride quality. Taking this into consideration, the inventors of the present invention have come to a finding which is explained below. If side forces act on the lower arms under conditions where the motor vehicle is in a cornering maneuver, there arises a compressive force exerted on each of the lower arms. The inventors have found that this compressive force produces a large amount of stress applied to the lower arms, causing the lower arms to bend downward, due to the presence of the recessed coil spring seat portions accommodating the lower ends of the coil springs of which length have been increased for improving the riding comfort.
More specifically, the center of stiffness of each lower arm is significantly lowered due to the presence of the recessed coil spring seat portion and is greatly offset downward from a centerline of load applied by the compressive force caused by the side force acting on each lower arm and, therefore, a great downward bending force is exerted on each lower arm.
In addition to this downward bending force, a reaction force exerted downward by the coil spring due to rolling motion of the vehicle acts on one of the lower arms during cornering, so that the lower arm is subjected to a considerably large downward bending force when the side forces are present. Conventionally, the lower arms have been made so large-sized that the lower arms would not deform due to the bending force exerted thereupon. In addition, the conventional lower arms would become so heavy to provide desired stiffness that the coil springs must have a high modulus of elasticity to suppress vibrations and oscillation of the heavy lower arms. For this reason, it has not been possible to achieve the desired level of ride quality with the conventional lower arm structure even though the length of the coil springs is increased to realize enhanced riding comfort.